In certain industrial or commercial processes, such as the drawing and crimping of synthetic tow and the like, and in other similar industrial processes, or in such commercial installations as a cooking unit in a restaurant kitchen, a substantial amount of steam, vapors, smoke, and/or oil fumes, hereinafter sometimes referred to as "exhaust gases", are generated. It is, of course, necessary to collect these contaminated gases and not let them escape into the atmosphere of the work room surrounding the machine. It has become commonplace to use an exhaust hood covering such work areas, which exerts a large negative pressure zone at the entrance thereto to draw in large quantities of air and insure collection of all contaminated exhaust gases emitted.
Until recent years, the large intake of contaminated air did not cause any particular problems, because the collected exhaust gases could be passed directly into the exterior air or atmosphere surrounding the industrial plant without control of pollutants. However, with the advent of the environmentalists, and new laws concerning release of contaminated exhaust gases into the atmosphere, it has become necessary to clean all the dirty exhaust gas prior to its introduction into the atmosphere. Such gas separating equipment includes air washers, cyclone separators, scrubbers, mechanical and electrostatic filters, and the like, which are fairly sophisticated and expensive. It is important to note here that the size or capacity of such air cleaning equipment is determined by the amount of air moving therethrough, not by the relative cleanness or contamination of such air. In other words, if it is necessary to withdraw ten thousand cubic feet per minute of air from the work station to insure collection of all exhaust gas emitted at the work station, it makes no difference as to capacity whether the gases withdrawn are contaminated with five parts per million of contaminated particles or one hundred parts per million of contaminated particles. The equipment will clean dirty air as easily as mildly dirty air and must be selected on the basis of the collection of ten thousand cfm, not on the basis of the relative contamination of the air. Therefore, it frequently occurs that expensive pollution abatement equipment may be required to cleanse air that is not relatively dirty.
Further, when large quantities of air are withdrawn from the area above the work table, this air must be replenished in some manner. Resultingly it is the conditioned room air which replaces the air withdrawn in a conventional exhaust hood system. This air must be replenished by similarly conditioned air in the room around the work area.
Also, with negative pressure or suction, it is relatively difficult to collect all of the contaminated exhaust gas emitted some two to three feet below the inlet to the exhaust hood. Some of the exhaust gases are lost through the side opening(s) before reaching the relatively high "capture point", which is the point at which the exhaust gases are sufficiently influenced by the suction from the air inlet of the exhaust hood to insure that they are drawn into the exhaust hood.